(1) Field of Invention
This invention relates to the synthesis of crystalline, porous inorganic oxide materials possessing uniform framework-confined mesopores in the range 2.0-10.0 nm and large elementary particle size of greater than 500 nm. In particular, the present invention relates to such materials where the formation of the mesoporous structure is accomplished by a novel self-assembly mechanism involving complexation and/or hydrogen (H) bonding between aqueous or alcoholic emulsions of various nonionic polyethylene oxide based surfactants (N.degree.) and various neutral inorganic oxide precursors (I.degree.). This is followed by hydrolysis and subsequent condensation of hydrolysis products at ambient reaction temperatures. This (N.degree. I.degree.) templating approach allows for the removal of template through calcination or solvent extraction which lowers material and energy costs. The template is biodegradable. The (N.degree. I.degree.) templating approach also affords non-lamellar mesostructures of metal oxides in addition to silica.
(2) Description of Prior Art
Modern human activities rely greatly upon porous solids of both natural and synthetic design. The pore structures of such solids are generally formed during crystallization or during subsequent treatments. These solid materials are classified depending upon their predominant pore sizes: (i) microporous, with pore sizes &lt;1.0 nm; (ii) macroporous, with pore sizes exceeding 50.0 nm; and mesoporous, with pore sizes intermediate between 1.0 and 50.0 nm. Macroporous solids find limited use as adsorbents or catalysts owing to their low surface areas and large non-uniform pores. Micro- and mesoporous solids however, are widely utilized in adsorption., separation technologies and catalysis. There is an ever increasing demand for new, highly stable well defined mesoporous materials because of the need for ever higher accessible surface areas and pore volumes in order that various chemical processes may be made more efficient or indeed, accomplished at all.
Porous materials may be structurally amorphous, para-crystalline or crystalline. Amorphous materials, such as silica gel or alumina gel, do not possess long range crystallographic order, whereas para-crystalline solids such as .gamma.- or .eta.- alumina are semi-ordered, producing broad X-ray diffraction peaks. Both these classes of materials exhibit very broad pore distributions predominantly in the mesoporous range. This wide pore distribution however, limits the effectiveness of catalysts, adsorbents and ion-exchange systems prepared from such materials.
Zeolites and some related molecular sieves such as; alumino-phosphates and pillar interlayered clays, possess rigorous uniform pore sizes. Zeolites are highly crystalline microporous aluminosilicates where the lattice of the material is composed of IO.sub.4 tetrahedra (I=Al, Si) linked by sharing the apical oxygen atoms. Cavities and connecting channels of uniform size form the pore structures which are confined within the specially oriented IO.sub.4 tetrahedra (Breck, D. W., Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley and Sons; London, pages 1 to 100 (1974)). Zeolites are considered as a subclass of molecular sieves owing to their ability to discriminate small molecules and perform chemistry upon them. Molecular sieves in general are materials with crystalline frameworks in which tetrahedral Si and/or Al atoms of a zeolite or zeolitic lattice are entirely or in part substituted by other atoms such as B, Ga, Ge, Ti, Zr, V, Fe or P. Negative charge is created in the zeolite framework by the isomorphous substitution of Si.sup.4+ ions by Al3+ or similar ions. In natural zeolites, this charge is balanced by the incorporation of exchangeable alkali or alkaline earth cations such as Na.sup.+, K.sup.+, Ca.sup.2+. Synthetic zeolites utilize these and other cations such as quaternary ammonium cations and protons as charge balancing ions. Zeolites and molecular sieves are generally prepared from aluminosilicate or phosphate gels under hydrothermal reaction conditions. Their crystallization, according to the hereafter discussed prior art, is accomplished through prolonged reaction in an autoclave for 1-50 days and oftentimes, in the presence of structure directing agents (templates). The correct selection of template is of paramount importance to the preparation of a desired framework and pore network. A wide variety of organic molecules or assemblies of organic molecules with one or more functional groups are known in the prior art to provide more than 85 different molecular sieve framework structures. (Meier et al., Atlas of Zeolite Structure types, Butterworth, London, pages 451 to 469 (1992)).
Recent reviews on the use of templates and the corresponding structures produced, as well as the mechanisms of structure direction have been produced by Barrer et al., Zeolites, Vol. 1, 130-140, (1981); Lok et al. , Zeolites, Vol. 3, 282-291, (1983); Davis et al., Chem Mater., Vol. 4,756-768, (1992) and Gies et al., Zeolites, Vol 12, 42-49, (1992). For example, U.S. Pat. No. 3,702,886 teaches that an aluminosilicate gel (with high Si/Al ratio) crystallized in the presence of quaternary tetrapropyl ammonium hydroxide template to produce zeolite ZSM-5. Other publications teach the use of different organic templating agents and include; U.S. Pat. No. 3,709,979, wherein quaternary cations such as tetrabutyl ammonium or tetrabutyl phosphonium ions crystallize ZSM-11 and U.S. Pat. No. 4,391,785 demonstrates the preparation of ZSM-12 in the presence of tetraethyl ammonium cations. Other prior art teaches that primary amines such as propylamine and ipropylamine (U.S. Pat. No. 4,151,189), and diamines such as diaminopentane, diaminohexane and diaminododecane (U.S. Pat. No. 4,108,881) also direct the synthesis of ZSM-5 type structure. Hearmon et al (Zeolites, Vol. 10, 608 -611, (1990)) however, point out that the protonated form of the template molecule is most likely responsible for the framework assembly.
In summary, most of the zeolites and molecular sieve frameworks taught in the prior art are assembled by using quaternary ammonium cations or protonated forms of amines and diamines as templates.
The need for new and useful types of stable frameworks and the need to expand the uniform pore size into the mesopore region allowing the adsorption and discrimination of much larger molecules, has driven the search for organic structure-directing agents that will produce these new structures. In the prior art however, molecular sieves possess uniform pore sizes in the microporous range. These pore sizes and therefore the molecular sieving abilities of the materials are predetermined by the thermodynamically favored formation of framework windows containing 8, 10 and 12 I-atom rings. The largest pore size zeolites previously available were the naturally occurring faujasite (pore size 0.74 nm) or synthetic faujasite analogs, zeolites X and Y with 0.8 nm pore windows (Breck, D. W., Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley and Sons; London, pages 1 to 100 (1974)). The innovative use of aluminophosphate gels has allowed the synthesis of new large pore materials. Thus, an 18 I-atom ring aluminophosphate molecular sieve; VPI-5 (Davis et al., Nature, Vol. 331, 698-699, (1988)) was produced and found to consist of an hexagonal arrangement of one dimensional channels (pores) of diameter .apprxeq.1.2 nm. A gallophosphate molecular sieve cloverite, with pore size of 1.3 nm was reported by Estermann M. et al (Nature, Vol 352,320-323, (1991)), while recently, Thomas J. M. et al (J. Chem. Soc. Chem. Commun., 875-876, (1992)) reported a triethyl ammonium cation directed synthesis of a novel 20 I-atom ring aluminophosphate molecular sieve (JDF-20), with uniform pore size of 1.45 nm (calculated from lattice parameters). A vanadium phosphate material was very recently reported with 1.84 nm lattice cavity (Soghmonian et al., Agnew. Chem. Int. Ed. Engl., Vol. 32,610-611, (1993)). However, the true pore sizes of the latter two materials are unknown since sorption data were not made available and furthermore, these materials are not thermally stable.
In summary, in spite of significant progress made toward the preparation of large pore size materials, thermally stable molecular sieves are still only available with uniform pore sizes in the microporous range.
A recent breakthrough in the preparation of mesoporous silica and aluminosilicate molecular sieves was disclosed in U.S. Pat. Nos. 5,098,684; 5,102,643. The class of mesoporous materials (denoted as M41S) claimed in this prior art was found to possess uniform and adjustable pore size in the range 1.3-10.0 nm. These materials exhibited framework wall thickness from 0.8 to 1.2 nm and elementary particle size generally greater than 50.0 nm. By varying the synthesis conditions, M41S materials with hexagonal (MCM-41), cubic (MCM-48) or layered morphologies have been disclosed (Beck et al., J. Am. Chem. Soc., Vol. 114, 10834-10843, (1992)). The mechanism proposed for the formation of these materials involves strong electrostatic interactions and ion pairing between long chain quaternary alkyl ammonium cations, as structure directing agents, and anionic silicate oligomer species (U.S. Pat. No. 5,098,684). Recently, Stucky et al (Nature, Vol. 368, 317-321 (1994)) extended this assembly approach by proposing four complementary synthesis pathways. The direct co-condensation of anionic inorganic species (I.sup.-) with a cationic surfactant (S.sup.+) to give assembled ion pairs (S.sup.+ I.sup.-), for example MCM-41, was described as Pathway 1. The charge reversed situation with an anionic template (S.sup.-) being used to direct the assembly of cationic inorganic species (I.sup.+) to ion pairs (S.sup.-, I.sup.+) was Pathway 2. Hexagonal iron and lead oxide and lamellar lead and aluminum oxide phases have been reported using Pathway 2 (Stucky et al. ibid.). Pathways 3 and 4 involve the mediation of assemblies of surfactants and inorganic species of similar charge by oppositely charged counterions (X.sup.+ .dbd.Cl.sup.-, Br.sup.-, or M.sup.+ .dbd.Na.sup.+, K.sup.+). The viability of Pathway 3 was demonstrated by the synthesis of hexagonal MCM-41 using a quaternary alkyl ammonium cation template under strongly acidic conditions (5-10 mol L.sup.-1 HCl or HBr) in order to generate and assemble positively charged framework precursors (Stucky et al. ibid). Pathway 4 was demonstrated by the condensation of anionic aluminate species with an anionic template (C.sub.12 H.sub.25 OPO.sub.3.sup.-) via alkali cation mediated (Na.sup.+, K.sup.+) ion pairing, to produce a lamellar Ai(OH)3 phase. Pinnavaia et al. (Nature, Vol 368, 321-323, (1994)) reported the preparation of a templated mesoporous silica and a Ti-substituted analogue by the acid catalyzed hydrolysis of an inorganic alkoxide precursor in the presence of primary ammonium ions.
All of the aforementioned synthetic pathways involve charge matching between ionic organic directing agents and ionic inorganic precursors. The template therefore, is strongly bound to the charged framework and difficult to recover. For example, in the original Mobil patent (U.S. Pat. No. 5,098,684) the template was not recovered, but burned off by calcination at elevated temperature. Template removal of anionic surfactant (Pathway 2) has however, been demonstrated by ion-exchange with low pH acidic cation donor solutions (U.S. Pat. No. 5,143,879). Template-halide pairs in the framework of acidic Pathway 3 materials can be displaced by ethanol extraction (Stucky et al. ibid). Thus, ionic template recovery is possible, provided that exchange ions or ion pairs are present during the extraction process.
Most recently, the formation of mesoporous molecular sieves via a new route (Pathway 5) was proposed by Pinnavaia et al. (Science , Vol. 267, 865-867, (1995)). In this method, the self assembly of micelles of neutral primary amines (S.degree.) and neutral inorganic alkoxide precursors (I.degree.) was based upon hydrogen bonding between the two components. The new approach (S.degree., I.degree.) taught in that prior art afforded mesostructures with greater wall thicknesses, smaller particle sizes and complimentary framework-confined mesoporosities relative to Pathway 1 and 3 materials. The new materials however, provided several advantages over the materials taught in the prior art. Greater wall thicknesses are desired in order that the thermal and hydrothermal stabilities of the materials may be improved (Coustel et al., J. Chem. Soc. Chem. Commun., 967-968, (1994)). Small particle sizes allow for greater volumes of textural mesoporosity in turn leading to greater access, via mass transport through the textural pores, to the framework-confined pores, thereby improving the overall performance of the adsorbent (Pinnavaia et al., ibid; Chavin et al., J. Catal., Vol. 111, 94-105, (1988)). In addition, owing to the weak template-framework interactions, Pathway 5 allowed for the facile solvent extraction of the template, removing the need for cation donors or ion pairs.
The terms framework-confined and textural porosity are herein defined. Framework-confined uniform pores are pores formed by the nucleation and crystallization of the framework elementary particles and are typically highly regular cavities and channels confined by the solid framework. The size of these cavities and channels is predetermined by the thermodynamically favored assembly routes. Textural porosity is that which can be attributed to voids and channels between elementary particles and/or aggregates of such particles (grains). Each elementary particle in the case of molecular sieves is composed of a certain number of framework unit cells each in turn containing framework-confined uniform pores. Textural porosity is formed during crystal growth and segregation or during subsequent thermal treatment or acid leaching. The size of the textural pores is determined by the size, shape and the number of interfacial contacts of these particles or aggregates. Thus, the size of the textural pores is generally one or two orders of magnitude larger than that of the framework-confined pores and is proportional to the elementary particle size.
One skilled in the arts of powder X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and adsorption/desorption can determine the existence of and differentiate between framework-confined and textural mesoporosities. The crystallographic distance between repeat units in the elementary particles and some information about the arrangement of such repeat units can be obtained from XRD. Particle sizes and shapes and preliminary information regarding textural mesoporosity can be established by SEM and TEM. Analysis of the N.sub.2 or Ar adsorption-desorption isotherms of the solid material can indicate both framework-confined and textural mesoporosities. Textural mesoporosity is evidenced by the presence of a Type IV isotherm exhibiting a well defined hysteresis loop in the relative pressure region P.sub.i /P.sub.0 &gt;0.5 (Sing et al., Pure Appl. Chem., Vol. 57, 603-619, (1985)). This behavior is common for a variety of para-crystalline materials and freeze-dried pillared layered solids. Framework-confined mesoporosity is characterized by a sharp adsorption uptake followed by a hysteresis loop in the 0.3-0.4 P.sub.i /P.sub.O region. This hysteresis corresponds to capillary condensation in the framework-confined mesopores. In MCM-41 materials, the large particle size precludes the formation of textural mesoporosity and a corresponding ratio of textural to framework-confined mesoporosity approaching zero is calculated. In materials prepared via Pathway 5, the elementary particle size was smaller (&lt;40.0 nm) producing a ratio of textural to framework-confined mesoporosity greater than 0.2.
In summary, according to the prior art, the molecular sieve materials and preparation techniques provide several distinct disadvantages and advantages:
i) The prior art of Pathways 1 through 4 teaches the use of charged surfactant species as templates in order to assemble inorganic frameworks from charged inorganic precursors. These charged templates are generally expensive, strongly bound to the inorganic framework and therefore difficult to recover. Additionally, many of these templates such as the most commonly used quaternary ammonium cations are highly toxic and environmentally undesirable. In the prior art of Pathways 1 to 4, the template was removed from the structure by either calcining it out or by ion-exchange reactions. Pathway 5 prior art templates are also highly toxic and environmentally unsuitable, but may be removed through environmentally benign ethanol extraction and thereby recovered and reused. PA1 ii) Prior art mesoporous molecular sieves produced by Pathways 1-4 exhibit small pore-wall thicknesses (0.8-1.2 nm), to which may be related the very poor thermal and hydrolytic stabilities of the materials taught in that prior art, while Pathway 5 provides materials with greater wall thicknesses (2.0 nm) and thereby greater stabilities. This contrast is ascribed to the differences in the self-assembly mechanisms with the former prior art relying on strong ionic interactions and the latter relying on weaker H-bonding interactions. PA1 iii) The prior art of Pathways 1-4 produces materials with low textural to framework-confined mesopore ratios, while the prior art of pathway 5 exhibits higher textural to framework-confined mesopore ratios and therefore, theoretically better access to the framework pores. However, the very small elementary particle size means that few pores are contained within any one particle, thereby theoretically producing lower specific activities.
There is a need for new methods of preparation of new materials of these types, cost reductions, ease of recoverability and environmental compatibility in the template and inorganic precursors has lead to the development of a new synthetic method to be described herein.